1. Field of the Invention
The present invention relates to a display device having a substrate defined by a plurality of layers including a reinforcing layer, and a manufacturing method thereof.
2. Description of the Related Art
With recent progress in communication technology, display devices such as a liquid crystal display device capable of driving with low power consumption and a self light-emitting organic EL display device have been developed as display devices for use in, for example, portable information terminal equipment. Reduction in weight and thickness, improvement in shock resistance, and the like have been demanded for display devices accordingly.
In order to respond to this demand, the use of a plastic substrate instead of a commonly used glass substrate has been proposed. However, a plastic substrate made of a sheet of a single resin has various problems as a substrate for display devices.
The most critical problem is that a plastic substrate has a relatively large linear expansion coefficient. In other words, glass generally has a linear expansion coefficient of about several ppm/° C., whereas plastic has a linear expansion coefficient of about several tens of ppm/° C. or higher, which is much higher than the linear expansion coefficient of glass. A substrate having a high linear expansion coefficient significantly varies in its dimensions with the ambient temperature. It is therefore extremely difficult to accurately pattern driving elements such as TFTs (thin film transistors) and the like. It can be considered to use a glass substrate having high dimension stability as a TFT substrate and to use a plastic substrate only as a counter substrate. In this case, however, it is difficult to accurately align a CF (color filter) formed on the counter substrate with pixel electrodes on the TFT substrate.
In order to achieve a reduced linear expansion coefficient and improved dimensional stability of a plastic substrate, it has been proposed to form a reinforcing layer containing a filler in a resin of a plastic substrate so that the resultant substrate becomes a composite material substrate as a whole. For example, a plastic substrate having a high heat resistance and a high rigidity can be obtained by including a fibrous material in the reinforcing layer (e.g., see Japanese Laid-Open Patent Publication No. H11-2812). Especially when the plastic substrate is a transparent substrate, it is preferable to use transparent fibers such as a glass cloth as the fibrous material.
On the other hand, a liquid crystal display device, for example, has a TFT substrate and a counter substrate which are laminated to each other through a frame-shaped seal member. Inside the seal member is enclosed a liquid crystal layer between the TFT substrate and the counter substrate. If a part of the seal member peels off from the substrate, a liquid crystal material flows out or a foreign matter enters the liquid crystal layer, degrading display quality.
In order to improve the adhesive power of a seal member to a substrate, it is known to form a substrate having a concave-convex surface in a seal-material formation region (e.g., see Japanese Laid-Open Patent Publication No. H04-20929). It is also known to form a substrate having a rough surface (e.g., see Japanese Laid-Open Patent Publication No. H03-55516). These methods aim to improve the contact power of the seal member by increasing the contact area between the substrate surface and the seal member.
In the case where a reinforcing layer containing a fibrous material is formed in a plastic substrate as described above, the reinforcing layer has a concave-convex surface corresponding to the concave-convex shape of the fibers. In order to improve smoothness of the surface of the reinforcing layer, it is necessary to further form a resin layer as a smoothness improving layer. Moreover, since it is generally extremely difficult to assign a function to reduce moisture permeability, oxygen permeability, and the like to a single resin, it is also necessary to separately provide an inorganic barrier layer for preventing moisture permeation and the like. As a result, the substrate having the reinforcing layer has a layered structure of a plurality of resin layers.
In the case where a plastic substrate has such a layered structure, however, the adhesion power between resin layers varies depending on the layers, whereby film peeling is likely to occur at an interface of the layers. This problem is particularly significant in a seal-member formation region which is likely to be subjected to an external force. In other words, in a display device having a substrate with such a layered structure, not only a seal member may peel off from the substrate, but film peeling is likely to occur in the substrate itself in the sealing-member formation region.
For example, in the case where the adhesion power between the seal member and the substrate surface is stronger than that between the layers at the interface, film peeling may occur at the interface of the layers rather than the seal member peeling off from the substrate. Moreover, in a baking process that is performed after the substrates are laminated to each other, film peeling may occur at the interface of the layers in the seal-member formation region due to the difference in expansion (thermal shrinkage) between the TFT substrate and the counter substrate.